1. Field of the Invention
The present invention relates to an oscillator for outputting signals having a plurality of different frequencies and a synthesizer tuner circuit and AM synchronous detect circuit using this oscillator.
2. Description of the Related Art
Reference frequency oscillators used in synthesizer tuner circuits and PLL (Phase Locked Loop) synchronous detectors for televisions, radio tuners and portable communications employ resonators such as crystal or ceramic resonators which are produced on a production line without adjustment.
This construction is shown in FIG. 9. Here, a voltage vo is converted to a current i1 by a voltage/current converter 41. This current i1 is fed into the desired resonator from within the n resonators X1 to Xn inside a resonator group 44 by making a selection with the switch 43. A voltage v1 is generated at the maximum amplitude for the oscillating frequency of the resonator at a phase difference of zero degrees and this is input to the amplifier 42. An oscillating loop can then be formed by taking the output voltage vo for the amplifier 42 as the input for the voltage current converter 41. The oscillating frequency of the resonator is taken as the reference oscillating frequency output.
In this way, in the case where a plurality of reference frequencies are necessary to maintain compatibility in countries and regions where intermediate frequencies differ, as with multi-system compatible televisions, only a number of resonators equal to the number of required frequencies are prepared. The oscillating frequency is then set up by making selections with the switch 43.
However, the reference frequency generators for the synthesizer tuner circuits and PLL synchronous detectors (AM synchronous detect circuits, FM synchronous detectors) used in televisions, radio tuners and portable communications are the most important elements with regards to deciding the performance and the circuit scale of the device.
Also, there is a pressing need to develop highly integrated, high performance IC's to achieve the substantial improvements in compact, lightweight, low cost circuit elements needed today. However, retaining the numerous functions which are required make this difficult. In addition to retaining performance when putting the reference frequency oscillator into integrated circuit form, there are also problems encountered in simplifying the peripheral circuits and reducing the number of adjustment processes.
With conventional reference frequency oscillators, a plurality of reference frequencies can be obtained without setting up the above-mentioned adjustment processes afresh. However, if a large number of reference frequencies become necessary, then the number of resonators is also increased. This will increase both the circuit scale and the parts costs. As the cost of the resonators increases with resonator precision, increasing the number of resonators has an extremely detrimental effect.
It is therefore not possible to construct a reference frequency oscillator using resonators which have a high performance and do not require adjustments in the manufacturing process without both the circuit scale and the parts costs being high. This means that IC circuits which combine high integration, high performance and low costs cannot be obtained.
The present invention has been made to resolve the above-mentioned problems and an object of the invention is to provide an oscillator, and a synthesizer tuner circuit and an AM synchronous tuner employing this oscillator, which uses only two resonators, does not require the setting up of any new adjustment processes and for which arbitrary frequencies can be set up.